Investigating the environmental and anthropogenic drivers of temperate reef grazers and developing a grazer-based metric for assessing reef health
Date
2025
Authors
Jones, Alec E.
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Abstract
Grazers structure ecosystems by consuming primary producers. Recently, human-induced predator loss has caused destructive overgrazing of foundational species by grazers. Kelp forests are one critical ecosystem that has been affected, where red sea urchin (Mesocentrotus franciscanus) grazers have overconsumed many areas, resulting in barren habitats. Importantly, not all grazers in these ecosystems remove the kelp canopy; species such as the red turban snail (Pomaulax gibberosus) merely graze on biofilms or remove small patches leaving the kelp blades intact. Given the disruptions (e.g., predator loss) to normal grazer controls, identifying key factors regulating grazer populations is crucial to inform conservation action. We investigated potential environmental and recreational fishing drivers of red sea urchin and red turban snail density on rocky reefs. We predicted that temperature and wave exposure would negatively affect grazer density, and that grazer density would be lower on loose seafloor. We also predicted lower urchin density and higher snail density in protected areas, where limited fishing promotes higher trophic levels and larger-bodied predators. We found a strong negative relationship between turban snail density and wave exposure, suggesting turban snails may be impacted by increases in wave energies. Conversely, urchin density was not significantly associated with any of the environmental variables tested, supporting that this species tolerates a wide range of environmental conditions and can maintain high densities over diverse regions without management intervention. Indeed, we found that protected areas with reduced fishing pressure returned significantly higher snail-to-urchin ratios, translating into healthier ecosystems and intact kelp forest communities. Reduced fishing pressure likely contributed to more complete predator communities in protected areas, which can improve urchin control, leading to increased macroalgal growth and snail densities. Our results support the role of protection from fishing as a key management strategy that can regulate damaging urchin populations and promote healthier reef communities.